1. Field of the Disclosure
Aspects of the present disclosure relate to ultrasonic transducers, and, more particularly, to methods of forming a piezoelectric micromachined ultrasonic transducer defining an air-backed cavity, and associated apparatuses.
2. Description of Related Art
Some micromachined ultrasonic transducers (MUTs) may be configured, for example, as a piezoelectric micromachined ultrasonic transducer (pMUT) disclosed in U.S. Pat. No. 7,449,821 assigned to Research Triangle Institute, also the assignee of the present disclosure, which is also incorporated herein in its entirety by reference.
The formation of pMUT device, such as the pMUT device defining an air-backed cavity as disclosed in U.S. Pat. No. 7,449,821, may involve the formation of an electrically-conductive connection between the first electrode (i.e., the bottom electrode) of the transducer device, wherein the first electrode is disposed within the air-backed cavity of the pMUT device, and the conformal metal layer(s) applied to the air-backed cavity for providing subsequent connectivity, for example, to an integrated circuit (“IC”) or a flex cable. In this regard, some prior art methods involved, for example, deposition of a conformal metal layer in the air-backed cavity of the pMUT in direct contact with the first/bottom electrode (see, e.g., FIG. 7A of U.S. Pat. No. 7,449,821). In another example, the conformal metal layer is deposited in a via formed in a dielectric film formed to expose the first/bottom electrode (see, e.g., FIG. 7B of U.S. Pat. No. 7,449,821). In yet another example, involving a silicon-on-insulator (SOI) substrate, the conformal metal layer is deposited in a via extending to immediately adjacent the transducer device (see, e.g., FIGS. 14 and 15 of U.S. Pat. No. 7,449,821). However, the formation of such vias (i.e., by etching) according to these exemplary prior art methods may be difficult due to, for example, the first/bottom electrode and/or the dielectric film being relatively thin, and thus providing an insufficient etch stop layer.
In other prior art methods, a device substrate may remain engaged with the pMUT device to provide a fixed thickness member for controlling the resonance frequency of the pMUT device. In such instances, an electrically-conductive connection between the first electrode and the conformal metal layer is formed through either a doped silicon layer (see, e.g., FIG. 7 of WO 2008/054395 A1, also assigned to Research Triangle Institute, wherein WO 2008/054395 A1 is also incorporated herein in its entirety by reference) or by a plug deposited in a via etched from the front side of the wafer into the silicon layer next to the piezoelectric (PZT) element (see, e.g., FIG. 14 of U.S. Pat. No. 7,449,821). However, such electrically-conductive connections may, in some instances, implement a lesser desirable conductor (i.e., doped silicon) connecting the first/bottom electrode to the conformal metal layer deposited in the via. More particularly, the doped silicon layer may demonstrate reasonable conductivity at high electric field levels, though the conductivity thereof may less desirably be nonlinear and markedly decrease at low field levels due to diode-like behavior in the doped silicon layer. In other instances, such electrically-conductive connections may involve contact between the first/bottom electrode and the conformal metal layer deposited in the via about a corner of the via (i.e., where the sidewall of the via meets the bottom or end wall of the via) and/or along a sidewall of the via. In such instances, it may be difficult to connect the conformal metal layer to the metal extending to the first/bottom electrode, since the sidewalls and/or corners of the vias may be rough or incompletely etched, thus possibly resulting in poor or inconsistent electrically-conductive engagement between the first/bottom electrode and the conformal metal layer.
Another aspects of some prior art methods is that the element forming the electrically-conductive connection between the first electrode and the conformal metal layer may be formed about one of the lateral edges of the pMUT device (see, for example, FIG. 15 of U.S. Pat. No. 7,449,821). In such instances, mechanical flexure of the actuated pMUT device may initiate or accelerate fatigue of the engagement between the electrically-conductive connection element and conformal conductive layer within the second via, for instance, due to stress concentrations about the sidewall/endwall edge of the second via. Such fatigue could result in failure of the electrically-conductive engagement therebetween and would thus create an open circuit condition between the first/bottom electrode of the pMUT device and the IC, flex cable or redistribution substrate engaged therewith. Further, having a different material (i.e., a metal) disposed about a lateral edge of the membrane of the pMUT device for providing the electrical connection could change the boundary condition for membrane flexing and thus affect the frequency and/or vibrational mode (i.e., the fundamental or harmonic mode) of the pMUT device, thereby adversely affecting the acoustic signals generated by the pMUT device.
Thus, there exists a need in the ultrasonic transducer art, particularly with respect to a piezoelectric micromachined ultrasound transducer (“pMUT”) having an air-backed cavity, for improved methods of forming an electrically-conductive connection between the first electrode (i.e., the bottom electrode) of the transducer device, the first electrode being disposed within an air-backed cavity of the pMUT device, and the conformal metal layer(s) applied to the air-backed cavity for providing subsequent connectivity, for example, to an integrated circuit (“IC”) or a flex cable.